The present invention relates to an improved method for preparing amine-formaldehyde microcapsules and, more particularly, to a novel dispersant useful therein. Still more particularly the invention relates to a novel method for preparing microcapsules containing a photohardenable composition.
Recently, a novel photosensitive material which employs microcapsules containing a photohardenable composition has been developed. This photosensitive material is described in its most basic form in U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead Corporation. A preferred photosensitive material is sensitive to visible light and is useful in reproducing full color images and is described in U.S. Pat. Nos. 4,772,530 and 4,772,541. A photosensitive material has also been developed by Fuji Photo Film Co. which uses a silver halide to initiate polymerization of the photohardenable composition in the microcapsules as described in European Patent Publication Nos. 0203613; 0228083; 0234580; and 0228053.
Images are formed using the aforesaid photosensitive materials by image-wise exposing a layer of photohardenable capsules to actinic radiation and rupturing the capsules by passing an imaging sheet containing the capsules through a pressure nip. In the most typical embodiments, the microcapsules contain a polyethylenically unsaturated monomer which polymerizes upon exposure. In the silver-based system, the photosensitive material must be heated after exposure to cause the photohardenable composition to harden. Due to the difference in the hardness of the capsules in the exposed versus the unexposed areas, only certain capsules release their contents when they are subjected to pressure in the pressure nip. If the internal phase contains a dye precursor, the dye precursor is image-wise released A color image is formed upon transferring the dye precursor to a developer layer. In previously disclosed embodiments, the developer layer may be present on the same support as the layer of capsules (U.S. Pat. No. 4,440,846) or on a separate support (U.S. Pat. No. 4,399,209).
One of the preferred methods for preparing microcapsules useful in the aforementioned photosensitive materials is by in situ polymerization of an amine-formaldehyde resin, more particularly, a melamine-formaldehyde resin. In these methods, the photohardenable composition and other constituents of the internal phase of the microcapsules are dispersed in water and an amine-formaldehyde prepolymer or the amine and the formaldehyde individually are added and the capsule is formed in situ by an acid catalyzed condensation reaction. Such methods are well known in the art, however, for optimum sensitometric performance in the aforementioned photosensitive materials, it has been found that the microcapsules should have a fairly uniform small particle size. A particle size of 4 to 8 microns has been suggested.
Previously, methods for preparing amine-formaldehyde microcapsules useful in the aforementioned photosensitive materials have been described in commonly assigned U.S. Pat. No. 4,608,330. In these methods the microcapsules are formed in an aqueous phase containing pectin as a viscosity modifier and a water soluble polymer such as isobutylene-maleic anhydride copolymer. More recently, methods have been described in U.S. application Ser. No. 128,292 filed Dec. 3, 1987 in which the aqueous phase contains pectin and sodium polystyrene sulfonate. U.S. Pat. No. 4,746,467 discloses a method for preparing amine-formaldehyde microcapsules in an aqueous solution of a water soluble copolymer of acrylic acid, styrene sulfonic acid, and an alkyl acrylate or methacrylate.
While these previous methods have been successful, they have not been completely satisfactory. Because pectin is a natural product its properties tend to vary seasonally as well as from one supplier to another. Due to the nature of sulfonation process, sulfonated polystyrenes also suffer a strong batch-to-batch variation. It would be desirable to design a method for producing microcapsules useful in the photosensitive materials described in the patents referenced earlier which have a uniform small size and which is not dependent on the use of pectin and/or sulfonated polystyrene. Attempts have been made to use other dispersants, such as hydroxy ethyl cellulose, polyvinyl alcohols, Caboset.RTM. materials, but they have not been satisfactory. To be useful as a dispersant the compound (1) must have suitable viscosity characteristics; (2) in the case of microcapsules containing silver based photohardenable compositions, must not consume alkali and must not invert the first silver halide/oil emulsion; (3) must not cause gelling or crosslinking upon storage; and (4) must have the appropriate surface activity to deposit the capsule wall at the oil/water interface.